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Night shifts and breast cancer ‘not linked’

Study refutes 2007 IARC findings

Emily Perryman | 30th May 2019

Night shift work does not increase the risk of breast
cancer, a UK study has concluded.

In 2007, the International Agency for Research on Cancer
(IARC) said shift work disrupting the body’s sleep-wake cycle was “probably
carcinogenic”. The agency is due to review its findings this summer.

According to the Guardian, Breast Cancer Now claims the
latest research, which it funded, is the most comprehensive to date.

Michael Jones, a co-author of the study and staff
scientist in genetics and epidemiology at the Institute of Cancer Research, in
London, said the new study found no overall link between women having done
night shift work in the last 10 years and their risk of breast cancer,
regardless of the different types of work they did involving night shifts, and
the age at which they started such work.

In 2009, in response to the 2007 IARC
research, Denmark began compensating dozens of women with the
disease who said their illness had been brought on by working night
shifts.

But the latest study, published in the British
Journal of Cancer, builds on the conclusions of a 2016
meta-analysis which suggested there was little or no association.

That study had been challenged due to the older average
age of participants and because it had limited details on the nature of women’s
shift work.

The median age of participants at the age of recruitment
in the latest study was 45 years, and 17.5% of participants reported being
night shift workers regularly working between 10pm and 7am within the previous
10 years. The shift data was followed up six years later.

The researchers observed that 2,059 out of 102,869 women
went on to develop invasive breast cancer. Taking into account confounding risk
factors, the researchers found no overall association with night shift work.

They also found no significant difference in risk
relating to the type of night shift work, the age at which the work was
started, or whether it began before or after a first pregnancy.

The only statistically significant trend was found
specifically with average night hours worked per week, but researchers said it
was not supported by previous evidence or any proposed biological explanation.

Delyth Morgan, chief executive at Breast Cancer Care and
Breast Cancer Now, said the findings will help to reassure the hundreds of
thousands of women working night shifts that it’s unlikely their job patterns
are increasing their risk of breast cancer.

Jones added that night shifts could still
have other adverse effects on health.